The scene opens in Claudio's prison cell, where the Duke is still traipsing around pretending to be a friar.
Claudio says he's prepared to die, but he still has hope that he might get to live.
The Duke attempts to comfort Claudio while preparing him for the possibility of death.
Claudio finally says he's ready for whatever happens.
The Duke spies on the brother and sister while Isabella reveals that Angelo has propositioned her.
At first, Claudio agrees that Isabella should not sleep with Angelo to save his life.
Isabella says she's so grossed out that she can't even "name" the thing that Angelo wants.
Isabella declares that she would gladly sacrifice her life to save her brother.
Claudio gets to thinking about things and decides that death is pretty scary. "Ay, but to die, and go we know not where; / To lie in cold obstruction and to rot," he muses.
(Hmm. Claudio sounds a lot like Hamlet in the famous "To be, or not to be" speech. Death, says Hamlet, is the "the undiscover'd country from whose bourn / No traveller returns" in Act 3, Scene 1 of Hamlet.)
Isabella cries "Alas, alas!"
We interrupt this program for a brain snack: We've seen plenty of great stage productions of these intense scene between Isabella and Claudio, but our favorite interpretation of this intense moment is artist William Holman Hunt's painting "Claudio and Isabella" (1850). Check it out here.
Claudio begs "Sweet sister, let me live," and Isabella screams "O, you beast! O faithless coward, O dishonest wretch!" and so forth. Isabella says she can't wait until he dies.
Claudio continues to beg, but Isabella refuses to listen.
The Duke, who has been spying the whole time, steps in and tells a fib – he says that Angelo didn't really mean it when he asked Isabella to sleep with him. Angelo was just testing her virtue, which means Claudio should prepare himself for death.
Alone with Isabella, the Duke cooks up a scheme to make everybody happy. Isabella should tell Angelo that she'll spend the night with him. Then, instead of meeting Angelo, Isabella will send Mariana in her place. It'll be dark so Angelo won't know that he's not getting it on with Isabella and he'll have to set Claudio free.
It turns out that Angelo was once engaged to Mariana but he broke up with her because she lost her wedding dowry when her brother's ship (and a bunch of money) sank at sea. When Angelo accidentally sleeps with Mariana, he'll have to marry her and Isabella can remain chaste.
Isabella thinks this is a terrific idea and runs off to tell Angelo that she wants to save her brother's life.
Note: In some editions of Measure for Measure (like the Norton edition), the action of the play continues in this scene. In most versions, like the Riverside Shakespeare (the one we're using here), the action picks up in Act 3, Scene 2.